1. Field of the Invention
The present invention relates to a semiconductor laser comprising an active layer of a Group II-VI compound for generating a biaxial compressive strain.
2. BACKGROUND ART
Recently, optical detection devices such as MO discs and printers have been in greater demand. To meet such a demand, a great deal of efforts have been made for developing semiconductor lasers.
As an example of such semiconductor lasers, there is a semiconductor laser made of a Group II-VI compound of the Periodic Table of Element semiconductor which is illustrated in FIG. 1. As shown in FIG. 1, the semiconductor laser has a multi-layered structure comprising a n.sup.+ -GaAs substrate 10, a n-Zn Se layer 11, a n-ZnSSe cladding layer 12, a CdZnSe active layer 13, a p-ZnSSe cladding layer 14, a p-ZnSe layer 15 and a p.sup.+ -GaAs layer 16, all layers being formed in order over the n.sup.+ -GaAs substrate 10. The CdZnSe active layer 13 forms a lattice alignment with the ZnSSe cladding layers 12 and 14 and thus have a double-heterostructure (DH structure). The active layer 13 and the cladding layers 12 and 14 can be formed by a crystal growth of ZnSe. The Group II-VI compound semiconductor laser with the above-mentioned structure operates in a manner that the active layer 13 and the cladding layers 12 and 14 are in a lattice alignment state. Such an operation manner is the same as the operation principle of the Group III-V compound semiconductor lasers which have been commonly used already.
Semiconductor laser oscillating a wave with a green color wavelength of wide use have been also developed which has an active laser for oscillating a wave having a green color wavelength of 480 nm to 550 nm. However, these semiconductor lasers have a difficulty in manufacture. That is, it is difficult to make such semiconductor lasers by using a semiconductor compound of a Group III-V compound. On the other hand, a Group II-VI compound semiconductor not only makes it difficult to make the semiconductor lasers, but also requires a complex refinement. In particular, the Group II-VI compound semiconductor is poor in optical gain characteristic which is the most important characteristic of semiconductor lasers, as compared with the Group III-V semiconductor compound. This makes it difficult to use practically the Group II-VI compound semiconductor. The Group II-VI compound semiconductor itself has an optical gain lower than a GaAs compound semiconductor by three times to five times under the same condition. As a result, it exhibits a high critical current density and thereby requires a current of several amperes for an operation at a room temperature. The most known techniques are at such a level that the semiconductor lasers are operated at a low temperature of about 700.degree. K. For achieving operations of semiconductor lasers at a room temperature, new methods capable of providing an improvement in optical gain are strongly needed.